A type-specific B-cell epitope at the apex of outer surface protein C (OspC) of the Lyme disease spirochete, Borreliella burgdorferi.

Abstract

Broadly protective immunity to the Lyme disease spirochete, Borreliella burgdorferi, is constrained by antibodies against type-specific epitopes on outer surface protein C (OspC), a homodimeric helix-rich lipoprotein essential for early stages of spirochete dissemination in vertebrate hosts. However, the molecular basis for type-specific immunity has not been fully elucidated. In this report, we produced and characterized an OspC mouse monoclonal antibody, 8C1, that recognizes native and recombinant OspC type A (OspC_A) but not OspC type B or K. Epitope mapping by hydrogen-deuterium exchange mass spectrometry (HDX-MS) localized 8C1's epitope to a protruding ridge on the apex of OspC_Aα-helix 3 (residues 130-150) previously known to be an immunodominant region of the molecule. Alanine scanning pinpointed 8C1's core binding motif to a solvent exposed patch consisting of residues K₁₄₁, H₁₄₂, T₁₄₃, and D₁₄₄. Analysis of 26 Lyme disease-positive serum samples confirmed human antibody reactivity with this region of OspC_A, with residues E₁₄₀ and D₁₄₄ as being the most consequential. Our results underscore the importance of α-helix 3 as a target of type-specific epitopes on OspC_A that should be taken into consideration in Lyme disease vaccine design.IMPORTANCEA central challenge in the development of vaccines against Lyme disease, the most common vector-borne infection in the United States, is the antigenically variable nature of the lipoproteins displayed on the surface of the disease-causing spirochete, Borreliella burgdorferi. For example, antibodies against one type of outer surface protein C (OspC), a lipoprotein involved in B. burgdorferi transmission and early stages of infection, may have little or no cross reactivity with another seemingly closely related variant of OspC, thereby hampering the use of a single OspC type as a vaccine antigen. For the sake of vaccine design, it is critical to identify the specific epitopes on OspC that both restrict and enable cross-reactivity.